Municipalities worldwide must contend with the safe disposal of sewage generated by households and businesses. Although sewage is mostly water, substantial quantities of organic solids and pathogenic bacteria, which thrive on such solids, are present. Treatment typically begins with aeration of the wastewater to begin breaking down these solids. The now activated sludge can be removed from the wastewater by clarifiers, the use of which are well known in the art, but it still contains pathogens and must be somehow rendered safe for disposal. Simply dumping waste can lead to the contamination of waterways, but burying it in a landfill is expensive and uses up limited landfill space. Environmental regulations also limit disposal methods and increase their potential cost.
One solution to the safe disposal of these organic wastes is the creation of compost. Proper composting in accordance with 40 C.F.R. §503.32(a)(7)(ii) and Appx. B(A)(4) kills pathogens through natural heating caused by the activity of aerobic bacteria. This process is, of course, less energy-intensive than generating sterilizing heat by burning fuel, and it is also superior environmentally to chemical means. The resultant end product, known as “Class A” compost, is no longer a threat to health, and in fact can be sold to the public for use as a natural fertilizer under 40 C.F. R. §503.15. This outcome is highly desirable because it turns sewage from a waste product requiring careful disposal into a commodity in demand.
Composting itself has been practiced for millennia, and probably predates recorded history. However, successful composting of sewage on an industrial scale has proven much more difficult than anticipated. Attempts have been made to adapt reactor tunnels designed for solid wastes to the more watery solids typical of sewage, with some disastrous results. The tunnels were designed to permit wastes to be inserted at one end and pushed forward by means of hydraulic rams. The tunnels are large enough that each day's waste consumes only a small fraction of tunnel space, permitting the waste to remain in the tunnel for a total of 30 days or more. As new waste was added at one end and forced forward, compost fell out of the other end. Fans or blowers were provided to force air through the pile to prevent the development of anaerobic bacteria.
This system was reasonably successful for solid organic wastes such as those collected from residential trash containers. When applied to sewage, however, the reactor tunnels faced two problems. First, aeration of the composting pile by means of fans or blowers proved difficult. The pile was essentially a non-porous mass which air could not penetrate. This tended to encourage the pile to become anaerobic, as aerobic bacterial were starved of the oxygen they needed to live. An anaerobic pile has an offensive odor that is impossible to eliminate. Second, the composting sewage ceased to move as a pile when pushed by the rams, and instead began to act more like a liquid, exerting hydraulic pressure on the floor, walls, and roof of the tunnel. This resulted in structural failures as tunnels literally burst like over-pressurized concrete balloons.
Previous efforts at the composting of sewage required fairly extensive dewatering of products by technical means. For instance, the article “Aerobic composting of waste activated sludge: Kinetic analysis for microbiological reaction and oxygen consumption” by Y. Yamada and Y. Kawase, Waste Management 26 (2006) 49-61, calls for sludge of 25% solid content, while “Recent advancements in Wastewater Sludge Composting” by Israil Turovskiy and Jeffrey Westbrook, Florida Water and Resources Journal, April 2003, 42-44, calls for 20% solid content, as well as the use of quicklime to increase sludge temperature. These comparatively dry inputs require additional steps such as centrifuges to remove water, which involve high capital costs and extra maintenance for plant operators. U.S. Pat. No. 4,659,472 calls for dewatering of a sludge/sawdust mixture, involving the addition of a polyelectrolyte to facilitate water removal by pressing.